The present invention relates to a method for manufacturing a dialkyl carbonate, and more particularly relates to a method for efficiently manufacturing a dialkyl carbonate from CO, O.sub.2, and an alcohol.
Because of their excellent impact resistance and other mechanical properties, as well as their excellent heat resistance, transparency, and so on, aromatic polycarbonates have come to be used as engineering plastics in a wide range of fields in recent years.
One method for manufacturing these aromatic polycarbonates that has been put to industrial use is a so-called phosgene process, in which an aromatic dihydroxy compound such as bisphenol is reacted with phosgene by interfacial polycondensation. Unfortunately, numerous problems with this method have been indicated, e.g., extremely toxic phosgene must be used, there is the question of what to do with the large quantity of by-product sodium chloride, and the methylene chloride that is normally used as a reaction solvent can pose health and atmospheric pollution problems.
One known method for manufacturing an aromatic polycarbonate besides the phosgene process is a method (melt method) in which an alkali metal compound such as sodium hydroxide is used as a catalyst in an ester exchange reaction between an aromatic dihydroxy compound and a carbonic diester. This method has garnered attention of late because it has the advantage of allowing an aromatic polycarbonate to be manufactured at lower cost, and it is preferable in terms of environmental safety since it does not involve the use of toxic substances such as phosgene or methylene chloride.
A diaryl carbonate such as diphenyl carbonate is used as the carbonic diester in the manufacture of a polycarbonate by this melt method. As discussed in Japanese Laid-Open Patent Application H9-194430, this diaryl carbonate is manufactured by an ester exchange reaction between a dialkyl carbonate and a hydroxyl group-containing hydrocarbon such as phenol. The dialkyl carbonate that serves as a raw material for this diaryl carbonate is manufactured from carbon monoxide, oxygen, and an alcohol, using a catalyst composed of a cupric halide such as cupric chloride.
For example, when methanol is used as the alcohol, dimethyl carbonate is manufactured by the following reaction. EQU 2CH.sub.3 OH+CO+1/2O.sub.2.fwdarw.(CH.sub.3 O).sub.2 CO+H.sub.2 O
The cupric chloride used as the catalyst here is surmised to form cupric methoxychloride by a primary reaction: EQU 2CuCl+2CH.sub.3 OH+1/2O.sub.2.fwdarw.2Cu(OCH.sub.3)Cl+H.sub.2 O
and to be regenerated by a secondary reaction: EQU 2Cu(OCH.sub.3)Cl+CO.fwdarw.(CH.sub.3 O).sub.2 CO+2CuCl.
The addition of a hydrohalic acid to the reaction system in order to improve the catalytic activity of the cupric halide used as the catalyst has been disclosed (see Japanese Laid-Open Patent Application H5-194327).
Nevertheless, with a method in which a cupric halide is used as a catalyst as above, the conversion rate at which the above-mentioned cupric alkoxychloride is formed is so low that the yield of the resulting dialkyl carbonate is not necessarily adequate, and furthermore some catalysts can clog the reaction tank and pipes, which is a problem in terms of manufacturing efficiency.
In light of this situation, the inventors conducted diligent investigation into a method for manufacturing a dialkyl carbonate more efficiently, and arrived at the present invention upon discovering that when
(i) a cupric halide and PA1 (ii) a compound capable of producing a copper halide alkoxide by reaction with a cupric halide are used together as a catalyst, the reaction proceeds in a state of sustained high catalytic activity, there is no clogging of the reaction tank and pipes by the catalyst, and a carbonic diester is obtained at a high yield. PA1 (i) a cupric halide; and PA1 (ii) a compound capable of producing a copper halide alkoxide by reaction with a cupric halide PA1 is used in the manufacture of a dialkyl carbonate using carbon monoxide, oxygen, and an alcohol as starting raw materials.
The present invention was conceived in light of the above prior art, and it is one goal thereof to provide a method for efficiently manufacturing a dialkyl carbonate from CO, O.sub.2, and an alcohol.